It is well known that alkali metal oxides tend to exert a fluxing action, and thereby facilitate melting, when included in a glass batch. Consequently, most commercial glasses have at least one alkali metal oxide included in their compositions to some extent. For example, the soda-lime silicate sheet glass, commonly used for structural and automotive glazing, usually contains about 18% soda (Na.sub.2 O).
The term "alkali metal" customarily refers to the metals in Group 1a of the Periodic Table. The present invention is generally effective with all such metals, but primary interest centers on sodium. Hence, the invention is largely described with reference to sodium ions, but is not so limited.
Even in glasses where soda is not intentionally included in the glass composition, it is normally found as an impurity. This is because of its wide-spread occurrence in glass batch materials. Alkali metal oxides, and soda in particular, are generally desirable as glass components. However, every effort must be made to minimize their presence in some glasses. Thus, an alkali metal ion content of as little as several parts per million may have deleterious effects. For example, silica can be sputtered on the surface of silicon chips to form a protective film. Very low concentrations of an alkali metal ion in the silica glass film can adversely affect the electrical properties of the chip in such an application.
The presence of alkali metal oxides, such as soda (Na.sub.2 O), tends to soften a glass, that is, to reduce its high temperature viscosity. It may also interfere with light transmission in fiber optic applications. In addition, as noted above, the mobility of alkali metal ions may seriously affect electrical properties.
It is known that treatment of a glass with a mineral acid, such as sulfuric acid or the gaseous component thereof, at least partially removes alkali metal oxides from a surface layer on the glass. Such treatment may be employed to improve glass weathering, reduce surface conductivity, and/or alter such properties as strength, refractive index and chemical durability.
United Kingdom Patent No. 948,309 proposes a chemical method of dealkalizing a silicate glass. The glass is treated at an elevated temperature with concentrated sulfuric acid (H.sub.2 SO.sub.4), or a liquid acid sulfate (e.g., Na.sub.2 HSO.sub.4), containing no more than 10% water. The reaction is described as an exchange of hydrogen for alkali metal. The hydrogen is capable of subsequent removal as water by heat treatment. The patent notes that electrolysis is not involved.
Techniques for altering a glass surface by ion exchange are also well known. This may be an exchange between alkali metals, or an exchange of an alkali metal for another metal, such as silver or copper. An early United States Patent, showing such an exchange promoted by an electric current, is U.S. Pat. No. 2,198,733 (Leibig et al.). Such an exchange may, of course, be beneficial, but it is an exchange of metals. It is not a removal of one metal without substitution of another metal.